Research On Grating Lobe Suppression Methods For Ultrawideband Sparse Array Based Through-the-Wall Imaging Radar

Transmitting low frequency eletromagnetic waves which can penetrate nonmetal obstacles like walls, ultrawideband through-the-wall imaging radar(TWIR) is becoming a promising technique with several widespread military and civilian applications, such as, antiterrorism, rescue and surveillance missions. With the advantage of real-time processing and low costs, multiple-input multiple-output(MIMO) array based ultrawideband radar systems can offer high resolution images making future work like detection and tracking easier. Regarding portability and to save the costs of TWIR systems, MIMO array often adopts sparse type with limited number of antennas. The inter-element spacing is large than a wavelength, and thus grating lobes will occur inevitably. Grating lobes are undesirable because they produce artifacts and contaminate images. So this dissertation is focused on the suppression of grating lobes brought by sparse array.This dissertation studies the beampattern of the one-way array, analyses the reason of the appearance of grating lobes according to the waveform, and points out that periodic and uniformly weighted one-way array has high resolution and low grating lobe level. Since radar uses a two-way array for imaging, two-way array is also studied and the effective array, which plays an important role in transforming a two-way array into a one-way array, is brought in. Different two-way arrays have different one-way effective arrays and thus various grating lobe levels, so grating lobes can be reduced by properly designing the weights and positions of antennas, namely array design. Back projection algorithm for stepped frequency continuous wave(SFCW) is introduced, and the image of each channel has different properties around the mainlobe and grating lobes is known, which lay the foundation for grating lobe suppression in imaging domain.Three kinds of array are researched and low grating lobe levels are achieved. First, the design method of MIMO array is studied, and the array can be designed according to the required grating lobe level and cross-range resolution. Then, for MIMO array with T/R antennas, methods for arrays with 4 antennas and with more than 4 antennas are proposed, and the latter is improved to enhance the efficiency of the array. At last, two-dimensional array is analyzed, two methods, one for one-way array, the other for two-array, are proposed. Both of them have lower grating lobe levels than previous methods.Then grating lobe suppression is researched in imaging domain. Coherence factor and phase coherence factor, which can suppress cross-range grating lobes but not range sidelobes, are introduced. So CF and PCF combined with dual apodization are proposed to combat range sidelobes and lateral grating lobes at the time. To further reduce grating lobes, extended CF and PCF using subbands of the stepped frequency continuous wave, are proposed, they can even reduce range sidelobes. Finally, grating lobe suppression methods using subapertures are investigated. The distribution of grating lobes after processed by two faulty methods, namely aperture difference and cross-correlation, are analyzed. Then the method that cross-correlation weighted aperture difference is proposed, which can suppress grating lobes sharply. These methods have been successfully applied to through-the-wall radar imaging(TWRI) to suppress grating lobes and high quality images are obtained.